This invention relates to an ultrasonic vehicle rangefinder, in particular, to a device which uses electromagnetic waves and ultrasonic waves and measures the distance to an object from the travel time of the latter, and also measures the relative speed between the objects and the vehicle.
There are already several ultrasonic rangefinders in existence, for example the one described in Utility Model Published Application No. Sho 57-68574. Such a device emits ultrasonic waves from an ultrasonic microphone and receives the pulses reflected from an object by means of a reception microphone. The distance is found from the phase difference between the emitted pulse and the received pulse.
However, in a previously existing ultrasonic rangefinder, if trouble should arise in distance measurement there is no way to tell if the trouble is in emission or reception. To troubleshoot such a device it is necessary to use a second ultrasonic rangefinder to aid in judging whether the trouble is in emission or in reception. This is troublesome.
In addition, when it rains there is the problem of mud getting on the transmitter or receiver, or both. In such a case it sometimes happens that no reflected waves are received and it is judged that something is wrong with the equipment. This is a particularly frequenct problem when, for example, the transmitter and receiver are mounted under the floor and aimed at the road to measure the speed of the vehicle (for example Utility Model Published Application Sho No. 56-665) and to measure the height of the vehicle and adjust it to road conditions.